The printed circuit board (also known as printed wiring board) is presently predominantly manufactured employing the contact printing principle.
For a circuit board of moderately complex circuitry and high packing density, the printed circuit board (PCB) is designed with the aid of a computer. Operating in an interactive mode a design engineer or a layout specialist develops a layout on a computer driven plotter wherein the position and routing of each interconnecting conductor is drawn for the purpose of visual checking and/or design finalization. When the check plot has been reviewed and approved, the circuit pattern is digitized and the coordinates of endpoints of conductors, conductor widths, coordinates of holes, etc., are encoded and stored in computer memory. A photographic film master is then generated using a computer-controlled precision photo plotter. Additional tooling is also produced for this pattern by the computer in the form of magnetic or paper tapes used to drive numerical controlled drilling machines, board testers and component insertion machines. The function thus described is commonly referred to as computer-aided design.
Manufacturing economy dictates that the PCB's be fabricated in panels with each panel containing multiple circuit boards. For example, a panel 18 inches by 24 inches may consist of six identical circuit boards and in the parlance of the art the panel is designed as six-up. To generate the six-up working film the digital information generated during the computer-aided design phase is employed to drill a bare board for a six-up pattern for use as a tooling panel. Six identical film patterns are then pasted up on the drilled tooling panel and visually registered to insure a perfect alignment between the drilled holes and their corresponding images. A new film master with the multiple PCB pattern is made from the paste-up and rechecked with the drilled tooling panel. The new film master represents a third generation image (second generation if the step-and-repeat operation is done by the photo plotter, an alternative approach if the plotter format is as large as the required panel size).
A large number of working films are then produced from contact prints of the new film master since each working film can only be used approximately 20 times before it is bleached and scratched beyond acceptable performance. This represents a fourth generation of image transcription.
The final transfer of circuit image onto the photoresist is done by manual paste-up and visual registration of the working film onto the predrilled and photoresist-coated panel and finally by exposure of photoresist with an ultraviolet light source. The panel is then developed, inspected and reworked, plated, etched and committed to further processing.
It should be clear from the heretofore description that the contact printing process requires, from the original digital information, many generations of image transfer. In each and all the transfer steps, meticulous care must be exercised to control the film dimensional stability through the strict control of ambient temperature and humidity. It requires a large staff for continually making films.
There are also other problems inherent in the preparation and transcription of circuit image by contact printing. One is the image degradation caused by film wear and worse still the problem of dust and particulate matter pickup during the handling and/or imaging processes. These minute, extraneous materials ultimately show up as electrical shorts or opens on the finished printed circuit boards. A large staff of quality assurance inspectors and touch-up operators is required for constantly examining and repairing the circuit images prior to plating and etching. Despite this, defective PCB's attributed to poor imaging are still produced.
What would be desired is to provide an improved PCB manufacturing process wherein substantial cost savings through manufacturing yield improvement by better imaging techniques could be realized; through significant reduction of staff for imaging work, inspection and touch-up operations; and through the accumulative savings of materials as a result of improved yields.